(1) Field of the Invention
This invention relates to an improved method for producing diarylmethanes. More particularly, the invention relates to a method for producing diarylmethanes using formalin, which diarylmethanes are useful as high boiling aromatic solvents and for various kinds of chemical products.
(2) Prior Art
Several methods have been proposed for producing diphenylmethane by condensing benzene with formaldehyde and for preparing dihydroxy diphenylmethane by condensing phenol and formaldehyde.
However, the handling of formaldehyde is difficult because it is a gaseous substance at ordinary temperatures. So that, as the material to be fed to a reaction system, paraformaldehyde and trioxane are practically used, which are in solid state at ordinary temperatures.
Meanwhile, the formaldehyde is usually supplied by makers in the form of an aqueous solution of formaldehyde, the so-called formalin. Therefore, in industrial production, the above-mentioned paraformaldehyde and trioxane are often prepared from the above aqueous solution that are supplied by makers. Accordingly, if it is possible to use the formaldehyde in the form of an aqueous solution for the reaction with the foregoing aromatic hydrocarbon, it is quite advantageous to reduce the cost for formaldehyde in industrial practice because it is possible to use the inexpensive material supplied by makers of formaldehyde.
However, this has been scarcely tried in the conventional art. One of the reasons is that, if an aqueous solution is supplied as a reaction material, the catalyst such as sulfuric acid and aluminum chloride that are useful for the condensation of formaldehyde, cannot be employed because they are water-soluble.
As a countermeasure to this problem, the reaction between phenol and formalin in the presence of solid acid catalysts such as zeolites is disclosed in Japanese Laid-Open Patent Publication No. 1-180835 and U.S. Pat. No. 4,895,988. These known methods are serviceable for the reaction of the aqueous solution of formaldehyde and phenols which are high in condensation activity with the formaldehyde. However, in the case of aromatic hydrocarbons such as benzene and toluene having low condensation activity, these methods are not always preferable in view of their low yields. Furthermore, any practicable method to condense benzene, toluene or xylene using formalin, has not been known.